At any rate, the change back to the semi-gloss paper lines up with the lifting of paper rationing, circa 1948.
Here are some corollary information from other sources (from a quick Google search) -
"The end of the war was accompanied by rapid inflation in all areas of the economy, and paper remained in short supply despite the end of rationing. It was not until September 1948 that all Modern Library titles were back in stock for the first time since the war." from https://www.ideals.illinois.edu/handle/2142/3714
"Since I collect juvenile series books, it is not uncommon to see a Grosset & Dunlap book featuring a "Wartime Conditions" notice on the title page or dust jacket.These appeared on G&D books from 1943 to 1945. The exact months is not quite certain though enough study has been made on the Hardy Boys and Nancy Drew books that it might be possible to get a rough estimate by making a study of the reference books for those.Grosset & Dunlap, as an example, started to use pulp paper (that turns brown and brittle with exposure to heat, sunlight, and air) starting in 1942 as white paper supplies dwindled. A typical Nancy Drew book was about 2 inches thick in 1941 and 1942. However, thinner paper was used over time so that by 1945 the books were only about 1 inch thick.The pulp paper continued to be used by Grosset & Dunlap through 1948. Only by 1949 was good paper being used on the juvenile series books." from https://www.librarything.com/topic/95897